Modeling aerosol  particles in the atmosphere has some major differences from modeling scalars such as water vapor or trace gases. For vapor and gases, a mass concentration or mixing ratio is sufficient. Particles on the other hand have a size distribution with diameters ranging over several orders of magnitude, from a few nanometers to many micrometers. To represent this size distribution, atmospheric modelers have resorted to two major paradigms. The first of these is a sectional approach in which the size distribution is subdivided into smaller sections covering a range of particle diameters. The approach has been very popular and has an obvious analogy with particle samplers which measure in size ranges such as a cascade impactor. The second approach assumes that there is a mathematical functional form which represents the size distribution. Moments of this distribution describe characteristics of the distribution. The zeroeth moment is the total number of particles, the second, and third moments are simply related to total surface area and volume respectively. Some modelers calculate even higher moments and related them to radiative information from remote sensing. Most of the presentation will concentrate on the second paradigm as implemented in a three-dimensional chemical-transport model, the EPA Community Multiscale Air Quality (CMAQ) modeling system.

Major needs for aerosol modeling include the following; (1) better estimates of material emitted from natural and anthropogenic activities at the earth's surface, (2) better representation of new particle formation in the atmosphere (this process is undergoing major revisions at this time and will need to be into models),  (3) improved representation of the production of organic particulate material by gas-phase chemistry, and (4) maintain consistency between the aerosol moments in the advection algorithms. These needs and issues concerned with modeling atmospheric aerosols can form the basis for the discussion session.